The present invention relates to a method of driving a liquid crystal display device of an opposing signal line structure in which active three-terminal elements, each of which having a gate electrode connected to a scanning line, a drain electrode connected to a pixel electrode and a source electrode connected to a reference line, are arranged on a first substrate, signal lines are arranged on a second substrate facing the first substrate, and an electric field is applied to a liquid crystal layer between the pixel electrodes and the second substrate.
In recent years, a liquid crystal panel has been often used as a display element of a word processor, personal computer, television set, etc. In order to produce such a liquid crystal panel, first, a number of films of metals, semiconductors or the like are formed on a light transmitting substrate such as glass. These films are patterned in a desired design by a photolithography technique to form two pieces of electrode substrates. The electrode substrates are then disposed to face each other and fastened with a predetermined space therebetween, and a liquid crystal is sealed in the space to provide the liquid crystal panel.
FIG. 11 shows a structure of a liquid crystal display device incorporating a generally used TFT (thin film transistor). Scanning lines 71, signal lines 72, TFTs 74 and pixel electrodes 75 are formed on a single glass substrate (first substrate). Moreover, as shown by an alternate lone and two sort dashes line, a common electrode 76 common to all pixels is formed on a surface of a glass substrate (second substrate, not shown), which surface faces the first substrate. The second substrate is disposed to face the first substrate with a liquid crystal layer (not shown) therebetween. Additionally, auxiliary capacitors CS (not shown) and reference lines (not shown) may be formed on the glass substrate (first substrate) having thereon the TFTs 74.
Regarding a driving method for providing a high-quality image with a liquid crystal display device of such a structure, for example, see xe2x80x9cDrive System for TFT-LCDs Using Digital Drivers having Gray-Scale Interpolative Function, Hisao Okada, the Journal of the Institute of Image Information and Television Engineers, Vol.51, No.10, pp. 1768-1776(1997), published October, 1997.
According to this reference, when a TFT is in an ON state, an equivalent circuit of a single pixel of a liquid crystal display device of the above-mentioned structure is as shown in FIG. 12(a). On the other hand, when a TFT is in an OFF state, the equivalent circuit is as shown in FIG. 12(b).
When the TFT changes from the ON state to the OFF state, the voltage of the pixel electrode is lowered due to the effect of a transition of a gate voltage through a gate-drain parasitic capacitance Cg. Such a change of the electric potential of the pixel electrode causes the apparent non-symmetry of the transmissivity of liquid crystal with respect to positive and negative drive voltages. Thus, a high-quality image display is prevented.
Therefore, in order to display a high-quality image on the liquid crystal display device, the above reference discloses conditions to be satisfied by the drive voltages of the scanning lines, signal lines and common electrode. More specifically, the conditions include that the average of the common electrode drive voltage is lower than the average voltage of the signal line drive voltages by a predetermined amount xcex94V, and the average voltage of the signal line drive voltages is increased with a decrease in the absolute value of a voltage to be applied to the liquid crystal (liquid crystal applied voltage), i.e., a decrease of the relative voltage difference between the signal line drive voltage and the common electrode drive voltage. The apparent non-symmetry of the transmissivity of the liquid crystal with respect to the positive and negative voltages are compensated by satisfying these conditions.
FIGS. 13 and 14 are given to explain the above contents. First, FIG. 14 shows the relationship between a common electrode drive voltage (common voltage) Vcom and a signal line drive voltage (gray-scale voltage) V0. In FIG. 14, V0A is a maximum value of V0, while V0B is a minimum value of V0. VcomH is a maximum value of Vcom, while VcomL is a minimum value of Vcom. As shown in FIG. 14, the average voltage of the common electrode drive voltage Vcom is lower than the average voltage of the signal line drive voltage V0 by xcex94V (xcex94V greater than 0).
Further, FIG. 13 shows the relationship between the common electrode drive voltage Vcom and four signal line drive voltages (V0, V2, V5 and V7) in respect of the phases and xcex94V, in accordance with the contents of the above reference. As shown in FIG. 13, the phases of V0 and V2 are inverted with respect to the phase of Vcom, while the phases of V5 and V7 are the same as the phase of Vcom. When Vcom is VcomL, among the whole signal line drive voltages, V0 applies a voltage V0A to the liquid crystal, while V7 applies a voltage V7A to the liquid crystal. V2 and V5 apply voltages (V2A, V5A) between V0A and V7A to the liquid crystal. Furthermore, when Vcom is VcomH, among the whole signal line drive voltages, V0 applies a voltage V0B to the liquid crystal, while V7 applies a voltage V7B to the liquid crystal. V2 and V5 apply voltages (V2B, V5B) between V0B and V7B to the liquid crystal.
Here, a gray-scale number is represented by n (n=0, 1, 2, . . . , 7), a liquid crystal applied voltage VLC is given by |Vnxe2x88x92Vcom|. For instance, V0Axe2x88x92VcomL. It is clear from FIG. 13 that the larger the gray-scale number n, the lower the liquid crystal applied voltage VLC.
Moreover, a curved line C1 in FIG. 13 connects the averages of the respective signal line drive voltages. Furthermore, FIG. 13 shows the average of Vcom by a straight light C2. It can be understood from the curved line C1 which rises toward the right that the greater the gray-scale number n, the higher the average of the signal line drive voltages and the larger the difference between the average of the signal line drive voltages and the average of Vcom.
Here, one reason why the results shown in FIGS. 13 and 14 are obtained is that the liquid crystal applied voltage VLC becomes lower as the gray-scale number n is increased, and consequently the amount of lowering of the voltage of the pixel electrode is increased. In other words, as the liquid crystal applied voltage VLC is lowered, the difference between the average of the positive and negative voltages of the liquid crystal applied voltage VLC and the average of Vcom as a reference is increased. Therefore, in order to minimize this difference, the technique disclosed in the above reference sets Vcom and the respective signal line drive voltages so that xcex94V is increased in accordance with the difference. More specifically, Vcom is set for each signal line drive voltage so that the average of Vcom is lower than the average of each signal line drive voltage by just an amount of xcex94V.
FIG. 15 shows a structure of a basic circuit corresponding to one output of a 3-bit digital driver (this circuit will be hereinafter referred to as a xe2x80x9cunit drive circuitxe2x80x9d). Data to be displayed is fetched in a sampling memory Msmp by a sampling pulse Tsmp, and then transferred to a holding memory MH by an output pulse LP. Next, the data stored in the holding memory MH is decoded in a decoder DEC. Then, an analog switch (ASW0, ASW1, . . . , or ASW7) corresponding to the value of the data is turned on, and the data is converted into a corresponding voltage and output as a signal line drive voltage (V0, V1, . . . , or V7). For instance, when the value of data is 0, the analog switch ASW0 is turned on, and the signal line drive voltage V0 supplied from an external device of the 3-bit digital driver is output to a corresponding signal line of the liquid crystal display device.
In general, one unit drive circuit is formed correspondingly to one signal line of the liquid crystal display device, and a collection of the unit drive circuits is generally called a driver. In FIG. 15, the voltages V0 to V7 are usually generated by an external circuit of the driver, and supplied to the driver. In general, a driver that generates these voltages is called a xe2x80x9cgray-scale power supplyxe2x80x9d, and its voltage is generally called a xe2x80x9cgray-scale voltagexe2x80x9d and serves as a signal line drive voltage. Namely, by setting the gray-scale voltage in the manner mentioned above, the signal line drive voltages are brought into the states shown in FIGS. 13 and 14.
Next, a schematic structure of the liquid crystal display device of the opposing signal line structure is illustrated in FIGS. 4 and 5. FIG. 4 is a perspective view, while FIG. 5 is a plan view. Here, a TFT is used as the active three-terminal element. On one substrate (first substrate), a gate electrode 17 of a TFT 14 is connected to a scanning line 11, a drain electrode 18 is connected to a pixel electrode 15, and a source electrode 19 is connected to a reference line 13 on the same substrate. The substrate on which the TFT 14 is formed will be hereinafter referred to as the xe2x80x9cTFT substratexe2x80x9d. As shown by the alternate long and two short dashes line of FIG. 5, formed on a substrate (second substrate) facing the TFT substrate is a signal line 12 made of a transparent conductor. In general, a transparent metal such as ITO (indium-tin oxide) is used as the transparent conductor which forms the signal line 12. Additionally, a liquid crystal layer is formed between the signal line 12 and the pixel electrode 15, and an electric field is applied to this liquid crystal layer. Such a structure is called the xe2x80x9copposing signal line structurexe2x80x9d.
As the liquid crystal display device having such an opposing signal line structure and a driving method thereof, Japanese laid-open patent application No. (Tokukaisho) 61-215590 (published Sep. 25, 1986, Zvi Yaniv et al., xe2x80x9cActive display addressable without crossed lines on a substrate and method of using the samexe2x80x9d) illustrates the structure in which the voltages of the reference lines 13 shown in FIG. 5 are all ground potential or connected by a common connection. According to this publication, a driving method will be explained. Specifically, in FIG. 16, a is a scanning line drive voltage (gate voltage) waveform, b is a signal line drive voltage (gray-scale voltage) waveform, and c is a common electrode drive voltage waveform (or reference line drive voltage waveform). Through a TFT which is switched on when the scanning line drive voltage (a) is high, the corresponding pixel electrode is charged by a relative voltage difference between the signal line drive voltage (b) and the common electrode drive voltage (c). In order to apply an AC voltage to the liquid crystal, it is necessary to invert the signal line drive voltage (b) with respect to the common electrode drive voltage (c).
Moreover, the above Japanese laid-open patent application No. (Tokukaisho) 61-215590 explains the decrease of the amplitude of the signal line drive voltage (b) by arranging the common electrode drive voltage (c) to have a rectangular wave. This is based on the same concept as the AC-driving of the common electrode of a liquid crystal display device having no opposing signal line structure. Since the AC-driving of the common electrode is disclosed in the above-mentioned reference xe2x80x9cDrive System for TFT-LCDs Using Digital Drivers having Gray-Scale Interpolative Function, Hisao Okada, the Journal of the Institute of Image Information and Television Engineers, Vol.51, No.10, pp.1768-1776 (1997), the explanation thereof will be omitted here.
When the liquid crystal display device of the opposing signal line structure is operated by a drive method which does not consider a lowering of the voltage of the pixel electrode, like a conventional structure, the above-mentioned apparent non-symmetry of the transmissivity of the liquid crystal with respect to positive and negative drive voltage occurs. Therefore, there is a possibility that phenomena such as flickering and image persistence appear, and a high-quality image display can not be provided.
On other hand, in the liquid crystal display device having the above-described conventional structure instead of the opposing signal line structure, the cause of the non-symmetry and the compensation method are proposed in the above-mentioned reference. It is therefore possible to compensate for the apparent non-symmetry of the transmissivity of the liquid crystal with respect to positive and negative drive voltages as disclosed in the reference, and consequently prevent the phenomena such as flickering and image persistence to provide a high-quality image display.
However, in the liquid crystal display device of the opposing signal line structure, since the structure is completely different, it is impossible to compensate for the non-symmetry by the method disclosed in the above-mentioned reference. Thus, there is a problem that a high-quality image display without defects such as flickering and image persistence can not be provided.
An object of the present invention is to provide a method of driving a liquid crystal display device of the opposing signal line structure, which is capable of achieving a high quality image display by compensating for the non-symmetry of the transmissivity of the liquid crystal with respect to positive and negative drive voltages to prevent flickering and image persistence.
In order to achieve the object, a method of driving a liquid crystal display device of the present invention is a method of driving a liquid crystal display device in which an active three-terminal element having a gate electrode connected to a scanning line, a drain electrode connected to a pixel electrode and a source electrode connected to a reference line is arranged on a first substrate, a signal line is arranged on a second substrate which faces the first substrate, and an electric field is applied to a layer of liquid crystal between the pixel electrode and the second substrate, and characterized by setting an average voltage of reference line drive voltages for AC-driving the liquid crystal to be higher than an average voltage of signal line drive voltages.
According to this structure, the average voltage of the reference line drive voltages for AC-driving the liquid crystal is set higher than the average voltage of the signal line drive voltages.
It is therefore possible to compensate for the apparent non-symmetry of the transmissivity of the liquid crystal with respect to positive and negative drive voltages. Hence, with the method of driving the liquid crystal display device of the opposing signal line structure, a high-quality image display can be achieved.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.